The present invention relates generally to controls for a variable speed air conditioner or heat pump system. More specifically, the present invention relates to a method and apparatus for insuring that the voltage on a large DC capacitor employed in the control of a DC motor driven variable speed air conditioning system is safely discharged when system power is interrupted or disconnected.
Heretofore, large power resistors, integral with variable speed air conditioning system motor drive electronics, have been switched across the large DC capacitor employed in the delivery of DC power to the system's DC motor-compressor to cause the capacitor to discharge when AC system power was lost. The purpose of the arrangement was to insure that when AC power to the system was interrupted or lost, the capacitor discharged to a safe level before service personnel attempted to service the equipment. Previously, the large motor control power resistors to which the capacitor was discharged had been integral with the drive electronics by which the system's motor-compressor was driven.
New electronically commutated motor drives have eliminated the need for the large (and expensive) power resistors. As earlier mentioned, the primary function of such power resistors related to the control and driving of the system's compressor. The discharge of the DC capacitor to the power resistors was a matter of convenience and, with respect to the power resistors themselves a secondary function. The elimination of the large power resistors by virtue of the advance in motor drive design has required that alternative discharge circuitry, external to the motor drive, be implemented for safely discharging the DC capacitor. Such circuitry, of necessity, requires the inclusion of a large resistor.
The need therefore exists to provide for capacitor discharge circuity, external of the motor drive in a variable speed air conditioning or heat pump system driven by an electronically commutated DC motor, preferably without the need to provide a discrete resistor dedicated to the capacitor discharge function.